1. Field of the Invention
This invention relates to the field of portable battery equipment, and more particularly, to a method and apparatus for charging portable batteries using synchronous rectification.
2. Description of the Related Art
To improve customer satisfaction with portable battery operated equipment, in particular notebook computers, it is desired to re-charge batteries as quickly as possible. The amount of time required to charge a battery depends on the chemical process as well as the battery charger power processing efficiency.
Re-chargers for portable batteries utilize switching regulators to regulate DC power input from a DC voltage source such as a battery or an AC to DC adapter. Switching regulators are typically classified into different configurations or xe2x80x9ctopologies.xe2x80x9d One such topology is the single-ended inductor circuit, consisting of relatively simple circuits where a switch determines whether the voltage applied to an inductor is the input voltage, Vdc, or zero. In this manner, the output voltage is a function of the average voltage applied to the inductor. The switch may be implemented using various electronic components, for example, a power transistor, coupled either in series or parallel with the load. The regulator controls the turning ON and turning OFF of the switch in order to regulate the flow of power to the load. The switching regulator employs inductive energy storage elements to convert the switched current pulses into a steady load current. Power in a switching regulator is thus transmitted across the switch in discrete current pulses.
In order to generate a stream of current pulses, switching regulators typically include control circuitry to turn the switch on and off. The switch duty cycle, which controls the flow of power to the load, can be varied by a variety of methods. For example, the duty cycle can be varied by either (1) fixing the pulse stream frequency and varying the ON or OFF time of each pulse, or (2) fixing the ON or OFF time of each pulse and varying the pulse stream frequency. Which ever method is used to control the duty cycle, the switch in switching regulators is either OFF, where no power is dissipated by the switch, or ON in a low impedance state, where a small amount of power is dissipated by the switch. This generally results in fairly efficient operation with regard to the average amount of power dissipated.
One method that has been utilized to improve operational efficiency of voltage regulators employs synchronous rectification. In synchronous rectification, a pair of switches, which are connected in series between the input voltage and ground, are synchronized so that either the input voltage or ground is applied to the input of an inductor. The synchronous control of the switches provides improved efficiency compared to traditional circuits which employed a switch and a diode.
Certain switching regulators with synchronous rectification provide a positive output voltage, however, current can flow out of, or into the regulator""s output. When input voltage is removed while current is flowing into the regulator""s output, energy stored in the inductor will be discharged, creating excess voltage in the circuit. This over-voltage condition frequently results in destruction of circuit components.
There are several ways to prevent damage from reverse current. U.S. Pat. No. 5,731,694 issued to Wilcox et al. teaches a method and circuit for controlling reverse current in switching regulators with synchronous rectification. The Wilcox et al. patent optimizes protection during low load current efficiency but does not pertain to battery charging applications. Further, when power is removed and then reapplied to the Wilcox et al. device, the current overshoots a steady state value. In battery charging applications, this overshoot can cause undesirable oscillations in protection circuits, where a protection switch is tripped ON and OFF for several seconds. The oscillations result in pulse charging which greatly lowers the efficiency of the charging process. Additionally, the current overshoot decreases the useful life of battery charger components, which are fabricated with graphite having a lattice structure that breaks down when exposed to over-current conditions.
In view of the foregoing, it is desirable to provide a switching regulator with synchronous rectification for use in a battery recharger, wherein the switching regulator includes a control circuit which reduces or substantially eliminates current overshoot.
In one embodiment, the present invention pertains to a computer system wherein a rechargeable battery circuit supplies power to a central processing unit, and the power is dissipated from battery cells in the battery circuit as the power is supplied to the central processing unit. A battery recharger includes a switching voltage regulator circuit that supplies current and voltage to recharge the rechargeable battery circuit. The rechargeable battery circuit determines when a fault condition is present in the battery cells and opens a charging switch to prevent the current and voltage supplied by the switching voltage regulator from being applied to the battery cells. When the fault condition is cleared and the charging switch closes, a limiting circuit generates a control signal to prevent the current from overshooting a steady state value. The limiting circuit utilizes a limiting function, such as a ramp function as input to a pulse width modulation comparator.
Another feature of the present invention is to provide a current amplifier in the limiting circuit that receives a current signal from the battery circuit and generates a current error signal that is input to the pulse width modulation comparator.
A further feature of the present invention is to provide a voltage amplifier in the limiting circuit that receives a voltage signal from the battery circuit and generates a voltage error signal, a current amplifier that receives a current signal from the battery circuit and generates a current error signal, and a power amplifier that receives a power signal from the battery circuit and generates a power error signal. The limiting circuit compares the voltage error signal, the current error signal, and the power error signal, and inputs the error signal having the greatest value to the pulse width modulation comparator.
In order to compare the error signals, a further feature of the limiting circuit includes an internal current source, a first transistor coupled to the current amplifier, a second transistor coupled to the voltage amplifier, and a third transistor coupled to the power amplifier. The first transistor, the second transistor, and the third transistor are coupled to the internal current source to determine the error signal having the greatest value.
Another feature of the present invention is to have a computer system which prevents reverse current flow in the switching voltage regulator by including a low current comparator that receives a signal indicative of the current flowing in the switching voltage regulator and generates an output signal, a transistor having a base coupled to receive the output signal from the low current comparator and to provide an input signal to a pulse width modulation comparator, wherein input signal maintains the current flowing in the switching regulator circuit above a threshold value.
In an alternative embodiment, an additional feature of the present invention is to provide an apparatus for controlling charge current in a battery recharging system. The apparatus includes a current amplifier receiving input indicative of charge current in a rechargeable battery circuit, and generating a current error signal, and a pulse width modulation comparator receiving a first input signal from a limiting function and a second input signal based on the current error signal.
A further feature of the present invention is to provide a voltage amplifier receiving input indicative of charge voltage in the rechargeable battery circuit, generating a voltage error signal, and outputting the voltage error signal to the pulse width modulation comparator.
A further feature of the present invention is to provide a power amplifier receiving a first power signal indicative of power in the switching voltage regulator and a second power signal indicative of power in the rechargeable battery circuit, generating a power error signal, and outputting the power error signal to the pulse width modulation comparator.
A further feature of the present invention is to provide a limiting circuit that compares the voltage error signal, the current error signal, and the power error signal to determine the error signal having the greatest value, and to generate an input signal to the pulse width modulation comparator based on the error signal having the greatest value.
A further feature of the present invention is to provide a limiting circuit that includes an internal current source, a first transistor coupled to the current amplifier, a second transistor coupled to the voltage amplifier; and a third transistor coupled to the power amplifier. The first transistor, the second transistor, and the third transistor are coupled to the internal current source to determine the error signal having the greatest value.
An additional feature of the present invention is to provide a limiting circuit that includes a low current comparator receiving a signal indicative of the current flowing in the switching voltage regulator, and generating an output signal, and a transistor coupled to receive the output signal from the low current comparator, and to generate a signal to maintain the current flowing in the switching regulator circuit above a threshold value.
In another embodiment, an additional feature of the present invention is to provide a method for controlling charge current in a battery recharging system wherein the battery recharging system includes a switching voltage regulator circuit coupled to a rechargeable battery circuit having a plurality of battery cells. The method includes: (a) determining when excess voltage is present in the battery cells; (b) opening a charge switch in the rechargeable battery circuit to prevent charge current from flowing to the battery cells when excess voltage is present the battery cells; (c) comparing current output by the switching voltage regulator circuit to charge current in the rechargeable battery circuit; and (d) limiting the current output by the switching voltage regulator to prevent overshoot in the charge current after the charge switch recloses.
An additional feature of the present invention is to provide a method for controlling charge current in a battery recharging system that further includes: (a) generating a voltage error signal based on the voltage output by the switching voltage regulator and charge voltage in the rechargeable battery circuit; (b) generating a current error signal based on the current flowing through a portion of the switching voltage regulator and charge current in the rechargeable battery circuit; (c) generating a power error signal based on the power input to the switching voltage regulator and charge power in the rechargeable battery circuit; (d) comparing the voltage error signal, the current error signal, and the power error signal to determine the error signal having the greatest value; and (e) limiting the current output by the switching voltage regulator based on the error signal having the greatest value.
The foregoing has outlined rather broadly the objects, features, and technical advantages of the present invention so that the detailed description of the invention that follows may be better understood.